This invention relates to the construction of high speed generators and, specifically, to a rotor containment shell designed to reduce windage losses.
More specifically, a high speed generator typically includes a rotor, stator, rotor support, system and other components. The rotor typically comprises a forging in combination with a permanent magnet or a wound field winding that generates the magnetic field. Outside the permanent magnetic or field winding, a containment shell, typically made of a metallic material combined with an organic fiber reinforced material (or a combination of the two), is required to contain the rotor components against centrifugal forces.
When air is used as a primary cooling media in high speed motors and generators where rotor peripheral velocities exceed 475 feet per second, windage losses becomes comparable to, or even larger than, electrical losses (when a smooth surface rotor is applied). Higher windage loss results in higher temperatures in the rotor body and cooling gas inside the air gap. Careful management of the windage loss is critical to insure the success of an air cooled machine. In the past, air-cooled arrangements have oftentimes not been adopted because windage loss was too high to meet efficiency targets. In such cases, air cooling was replaced typically by more costly water cooling arrangements. In small machines the space between the rotor and stator is evacuated to reduce windage. But this is only practical for water cooled machines of power ratings too small to be used in distributed power generation.
Axial flow results in a significant increase of windage loss. Studies show that as axial flow increases, the axial velocity gradient increases while the swirl velocity distribution remains similar. This results in an increase of windage.